ath9k_hw: remove unnecessary parts of the AR9380 SREV check

Older versions have not been sold and the driver does not explicitly
check for them anyway, so we can simply ignore the macRev here.
Reduces ath9k_hw size on mips by more than 2 KB.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>